Polymers of oxystyrene series primarily including hydroxystyrene are used as functional polymer materials in various industrial fields. In particular, such polymers are used in the field of electron materials, particularly as raw materials for resin components for semiconductor resists. Additionally, such polymers are now being investigated for use as photosensitive resin components for layer insulation films and surface protective films of semiconductor devices and the like. Improvement in the properties of oxystyrene-series polymers and imparting novel properties to oxystyrene-series polymers are desired due to the high integration and enlarged size of semiconductor devices, the thinner and downsized sealing resin packages, transfer to surface mounting with solder reflow and the like. In view of this, it is now attempted to introduce various structure units into oxystyrene-series polymers by copolymerizing oxystyrene-series polymers with monomers copolymerizable with the oxystyrene-series polymers.
For example, Patent Document 1 proposes a photosensitive resin composition obtained by using a hydroxystyrene-series ABA-type triblock copolymer which uses an elastomer block including butadiene and isoprene as the component B. Patent Document 1 describes that the thermal shock resistance of polyhydroxystyrene can be improved by introducing the elastomer block including butadiene and isoprene. However, the segment including butadiene and isoprene has poor solubility in developing solutions and poor adhesion to substrates, so the ratio of the segment could not be raised and the effect of improving the thermal shock resistance was not sufficient.
Alternatively, oxystyrene-series monomers primarily including hydroxystyrene have cationic polymerization reactivity, being greatly different from those of vinyl ether-series monomers. Hence, it has been very difficult to obtain a copolymer of an oxystyrene-series monomer and a vinyl ether-series monomer.
For example, Patent Document 2 describes a method for producing a narrowly dispersible copolymer of an oxystyrene-series monomer and a vinyl ether-series monomer by living cationic polymerization, using a halogenated organic compound and a metal halide with Lewis acidity as initiators. Due to the difference in polymerization profiles between the respective monomers, the ratio of the vinyl ether monomer in the copolymer cannot be raised and thus, the ratio of the vinyl ether-series monomer incorporated in the copolymer was about 15 mol % at the most. In Patent Document 2, further, an oxystyrene-series monomer and a vinyl ether-series monomer are preliminarily mixed together and made to react using a monofunctional initiator. In such system, it is considered that only the vinyl ether-series monomer with fast reactivity is polymerized first and then, the oxystyrene-series monomer is polymerized continuously. Once an oxystyrene-series monomer is added to the resulting polymer, the vinyl ether-series monomer with significantly different reactivity no longer reacts with the resulting product, so the polymer obtained is not a random copolymer but a diblock copolymer, meaning that ABA-type triblock copolymer could not be obtained. Due to the same reason, it was difficult to synthetically prepare an ABA-type triblock copolymer by sequential reactions of an oxystyrene-series monomer and a vinyl ether-series monomer using a monofunctional initiator.
Various approaches have been studied, including separately preparing a polystyrene-series polymer and polyvinyl ether under different polymerization conditions, subsequently introducing a substituent at the end of the polyvinyl ether, and linking the polystyrene-series polymer through the substituent to the polyvinyl ether.
For example, Patent Document 3 discloses a method for producing a block polymer comprising a polystyrene-series polymer component and a polyvinyl ether-series polymer component, comprising preparing polyvinyl ether having hydroxyl group at the end of the polymer by living cationic polymerization of vinyl ether using a polymerization initiator with hydroxyl group protected with trimethylsilyl group and the like and subsequently conducting cationic polymerization of styrenes using the resulting polyvinyl ether as a terminator. Even in Patent Document 3, it is only a diblock polymer of polystyrene and poly(t-butyl vinyl ether) that is specifically described about the synthesis thereof. There are no examples shown in Patent Document 3 of the synthesis of an ABA-type triblock copolymer of polyvinyl ether and an oxystyrene-series polymer. Additionally, a block copolymer obtained by using polyvinyl ether comprising hydroxyl group at the end of the polymer as a terminator is disadvantageous in that it is thermally unstable because the polystyrene-series polymer component and the polyvinyl ether-series polymer component are linked together through an ether bond.
Patent Document 4 discloses a method for producing polyalkenyl ether with thiol group at both ends of the polymer comprising living cationic polymerization of vinyl ether using alkenyl ether with thiocarbonyl ester bond at a side chain as an initiator and a specific thiocarboxylate salt or a thioester compound as a terminator. The polyalkenyl ether with thiol group at both ends as obtained by the above method are used as a chain transfer agent to polymerize various vinyl polymers which are radical-polymerizable to obtain an ABA-type triblock polymer having the polyalkenyl ether as the center block (as the component B). Patent Document 4 however does not show any specific synthetic example of the block polymer nor disclose any polymerization method of a ABA-type triblock copolymer of polyvinyl ether and an oxystyrene-series polymer. In case where the polyalkenyl ether with thiol group at the ends of the polymer is used as a chain transfer agent, the resulting ABA-type triblock copolymer is thermally unstable because the component A and the component B are bonded together through sulfur atom, so that the triblock copolymer is readily colored during drying or heating, which is disadvantageous. With the radical polymerization method, polymerization can be performed in a simple manner but the molecular weight distribution cannot be controlled. Hence, there is a problem that narrow polymer which is narrowly dispersible cannot be obtained by the method.
In any of the methods, it is essential to prepare the segment comprising polyvinyl ether and the segment comprising an oxystyrene-series polymer synthetically in separate steps. Thus, the methods were laborious.
As described above, there are no reports describing about a ABA-type triblock copolymer comprising polyvinyl ether and an oxystyrene-series polymer. Therefore, there is a demand for development of a thermally stable ABA-type triblock copolymer comprising polyvinyl ether as the center block and a simple process of producing the same.    Patent Document 1: JP-A-2004-240143    Patent Document 2: JP-A-2003-342327    Patent Document 3: JP-A-2001-19770    Patent Document 4: JP-A-H6-116330